Geology – overview

by Eileen McSaveney and Simon Nathan

New Zealand is part of a mostly submerged continent that broke away from the Gondwana supercontinent millions of years ago. As the new land moved into the Pacific Ocean, it gradually submerged, then was uplifted by activity in the earth’s crust. New Zealand’s geology records the story of the country’s birth and growth.

New Zealand – a geological jigsaw puzzle

The people who settled New Zealand came to a dramatic and
unquiet land, with rugged mountains, active volcanoes and
frequent earthquakes. It is a country with a complex
geological history. Its bedrock is a geological jigsaw
puzzle. An understanding of New Zealand’s past and its lively
geological activity has come only in recent decades, as
scientists have developed a theory of the global workings of
the earth’s crust – a concept called plate tectonics.

A mobile planet

Earth’s main features – its continents and ocean floors –
are not fixed. The earth’s surface is divided into about 15
major segments, or plates, that move slowly about on the
soft, plastic rock of the underlying mantle. At mid-ocean
ridges, molten material wells up to the surface and cools.
This creates a new sea floor that slowly moves away from the
ridges a few centimetres per year. Where two plates meet, old
parts of the ocean floor sink back down. Embedded in the
moving plates, the continents drift together or split apart,
constantly changing the geography of the planet.

Highlands of a sunken continent

Exploration of the sea floor has revealed that the land we
call New Zealand was once more extensive than it is today.
The long, narrow, mountainous landmass of New Zealand is
merely the highlands of a submerged continent called
Zealandia. Extending to the north-west of the country are
large, shallow continental shelf areas – the Challenger
Plateau and Lord Howe Rise. To the south-east are the Chatham
Rise and Campbell Plateau. Offshore drilling shows that the
same rock type that makes up much of the Southern Alps
underlies the continental shelves as well.

New Zealand – a collision zone

The mostly submerged New Zealand continent, Zealandia,
sits uneasily astride two moving segments of the planet’s
surface – the Pacific and Australian plates. In the North
Island, the boundary between the plates lies off the East
Coast along a depression, the Hikurangi Trough, at the edge
of the continental shelf. In the Marlborough region, the
boundary cuts diagonally across the South Island to the West
Coast. It then continues south-westward along the great
Alpine Fault, and runs back out to sea near Milford
Sound.

The two moving plates are colliding at a glancing angle.
In the process, the sunken New Zealand continent is crumpling
to form the land that now projects above sea level. In the
north, ocean floor at the surface of the Pacific Plate to the
east plunges beneath the continental shelf off the eastern
North Island. As it does so, it pushes up the overlying rocks
and sediments, creating the hilly terrain of the eastern
North Island. In the South Island, the two plates are
directly colliding along the Alpine Fault. This causes a much
greater uplift, forming the Southern Alps.

At the same time, the country is being wrenched apart.
Along the Alpine Fault in the South Island, the West Coast
region west of the plate boundary is moving north-east at 2–3
metres per century, relative to the Southern Alps on the
eastern side. As this movement continues in the future, the
South Island will become more elongated.

Rocky foundations

South Island and Stewart Island

Large areas of the South Island, including much of the
Southern Alps, consist of masses of drab grey sedimentary
rocks known as greywacke. Their layers and the rare fossils
they contain indicate they were once deposited on the sea
floor, but most are now folded and broken into huge slabs
that stand nearly on end, like books on a shelf. To the west
and south, these grey rocks are transformed into schist, a
rock with glittering minerals formed under high heat and
pressure.

Cutting across the South Island is a straight boundary
that is visible from space – the Alpine Fault. Immediately
east of the fault, the greywacke and schist of the Southern
Alps have been raised many thousands of metres. In the
regions west of the fault, in north-west Nelson, Fiordland,
and also on Stewart Island, the bedrock is quite different
and includes much older rocks, as well as masses of granitic
rock solidified from molten magma. Geologist Harold Wellman
realised that large areas of rocks in north-west Nelson
matched rocks in Fiordland and Otago. He proposed that these
rocks were once continuous. A major section of the country
has been wrenched apart and shunted over 480 kilometres along
the Alpine Fault over the last 20 million years.

North Island

The greywackes that make up the Southern Alps also
underlie large areas of the North Island, but they are
exposed only in mountains such as the Tararua and Ruahine
ranges. In the rest of the North Island, they are buried
under blankets of more recently deposited sedimentary rocks.
On the Coromandel Peninsula and in the north of the central
North Island, the greywacke is masked by great thicknesses of
volcanic rock.

The geological time scale

Earth’s history can be deciphered from layers of rock and
the remnants of life fossilised within them. Based on the
changing succession of fossils, scientists have split
geological time into major divisions called periods. Some
period names, such as the Jurassic, are familiar to many
people. The periods were originally based on sequences of
rocks found in Europe. One period ends and another begins at
the point when a sudden change can be seen in the rocks and
fossils that were being deposited.

Fossils show the changes in the plants and animals that
lived on the land and in the seas. Most fossils preserved in
New Zealand are slightly different to those in Europe, but
they show the same evolutionary changes and are similar
enough to be useful guides.

In New Zealand the major international time periods have
been subdivided into shorter spans of time – called series
and stages – and given local names, generally after the
places where the rocks formed during that time are best seen.
New Zealand is unusual in having one of the thickest and most
complete Cenozoic successions in the world.

How old are the rocks?

The age of rocks which have been crystallised from molten
material (igneous rocks), or altered by heat and pressure
(metamorphic rocks), can be determined by analysing the
breakdown of radioactive minerals in them. By dating
crystalline rocks of the land and the sea floor, and seeing
how these relate to New Zealand’s sedimentary rock layers,
scientists have worked out a basic outline of the country’s
geological history. The oldest rocks in New Zealand formed
about 510 million years ago, during the Cambrian period.

New Zealand’s oldest rocks

Earth is estimated to have formed about 4,600 million
years ago. All of earth’s major continents contain extensive
regions of bedrock that formed during the planet’s earliest
history, a time known as the Precambrian era. Many of these
rocks are thousands of millions of years old. New Zealand,
however, has no rocks from this early period.

A newcomer

The beginning of the Cambrian period, about 540 million
years ago, is marked by the appearance of the first
widespread fossil evidence of life. At this time earth looked
very different. Australia, Antarctica, India, Africa and
South America were all parts of a single, huge supercontinent
called Gondwana, which spanned the south of the southern
hemisphere.

New Zealand is a geological newcomer – its bedrock has
formed since the beginning of the Cambrian period. Much of
that time it was under construction on the fringes of
Gondwana. It is only in the last 85 million years that New
Zealand broke free of Gondwana and moved into the Pacific
Ocean.

The formation of New Zealand’s landmass

The greater part of the landmass of New Zealand – the area
above sea level and its extensive continental shelves – is
built from recycled material. The rivers of Gondwana carried
sediment into the ocean. These sediments built up offshore
for millions of years, until movements of the sea floor
carried them towards the land. There the sediments were
plastered onto the edge of the continent, creating new
coastal mountains that enlarged the land area of Gondwana.
The main part of the New Zealand landmass, sometimes called
the ‘basement’ rocks, was formed on the margins of Gondwana
during several of these cycles of deposition and mountain
building.

The oldest rocks – Cambrian to Devonian periods

The oldest section of New Zealand’s landmass was formed
during the Cambrian through to the Devonian periods, some 540
to 360 million years ago. They originally consisted of
sediments deposited on the sea floor offshore from the parts
of Gondwana that would later become Antarctica and Australia.
Volcanic activity on offshore islands also produced volcanic
rocks and sediment.

The oldest sedimentary rocks in New Zealand, found in the
Cobb valley, north-west Nelson, were deposited about 510
million years ago, during the Cambrian period. Their age is
known from the fossils they contain, including animals called
trilobites.

One of the most widespread older rocks, found throughout
the western side of the South Island, is a greenish-grey
greywacke called the Greenland Group. These greywackes were
deposited in early Ordovician times, about 480 million years
ago. Greenland Group sediments have been heated and
metamorphosed to dark-grey gneiss. Rocks similar to the
Greenland Group are found in other parts of former Gondwana,
including Antarctica and eastern Australia.

During the late Devonian and Carboniferous periods, the
sediments were disrupted by movements of the earth’s plates.
Sea floor movement carried them towards the Gondwana margin,
where they were squeezed and folded to form land that
eventually became part of Australia, Antarctica and New
Zealand. Many of the sediments that had been deposited in the
ocean were altered by heat and pressure to form metamorphic
rocks such as schist and gneiss. The heat was sufficient in
some areas to completely melt the rock, which recrystallised
to form large masses of granite and diorite. These rocks
today can be found along the West Coast of the South Island
from Fiordland to Nelson. The crystalline rocks are resistant
to erosion, and can form steep-walled valleys such as those
in Fiordland.

Building a continent

Until about 300 million years ago, much of the rock that
makes up New Zealand did not exist. Around this time,
sediment from the Australian and Antarctic sections of
Gondwana and its offshore islands began to accumulate in the
ocean.

Torlesse greywackes

Greywacke forms the mountain ranges in both the North and
South islands. This rather drab-looking rock consists of beds
of muddy grey sandstone alternating with thinner layers of
darker mudstone.

Greywacke occurs in other parts of the world. How it
formed remained a matter of debate until the 1960s, when
exploration of the deep ocean floors began. Large fans of
sediment were discovered on the sea floor at the foot of
valleys and canyons cut in the continental slopes. Sandy
sediment dumped by rivers onto the continental shelves
intermittently cascaded down the canyons as turbidity
currents – soupy mixtures of sediment and water – spreading
blankets of sand on the fans. During periods between the
turbidity currents, thin layers of mud settled slowly out of
the ocean and covered the sands.

Over 200 million years, tens of thousands of metres of
these sediments built up off the edge of Gondwana. They were
eventually buried, deformed and hardened to become the rocks
known as the Torlesse greywackes. Today, Torlesse rocks make
up more than half of the New Zealand landmass. They cover a
vast area, extending from Otago to East Cape, and below the
ocean across to the Chatham and Auckland Islands.

Source of the greywackes

The Torlesse greywackes, which are named after the
Torlesse Range of inland Canterbury, contain large amounts of
quartz and feldspar, the main minerals in granite. Detailed
studies of the mineral grains suggest that much of the
Torlesse greywacke is derived from granitic rocks in
north-east Australia.

Western Arc and Murihiku rocks

While greywacke sediments were accumulating far offshore
from the Gondwana supercontinent, sediments of a quite
different type were being deposited in shallower coastal
waters. Stretching for more than 1,000 kilometres along
Gondwana’s eastern coastline was a chain of volcanic islands.
For nearly 200 million years, ash from eruptions and sediment
from their erosion built up on the sea floor. The layers
hardened to form the Western Arc and Murihiku rocks. These
rocks once formed a continuous band, but have been separated
by later movement along the Alpine Fault. They are now found
in Southland and from east Nelson to South Auckland.

Disturbances on the edge of Gondwana

The sea floor is constantly on the move. About 250 million
years ago it began to shunt offshore sediments towards the
coast of Gondwana. As the sea floor was pushed beneath the
edge of the supercontinent, the piles of sediment on top were
broken into sections and scraped off. They piled up as stacks
of steeply dipping, overlapping slabs. Slices of ocean floor
were also caught up in the collision. Eventually the rocks
were thrust above the surface to form a mountainous new area
of land on the edge of Gondwana.

Magnetic disturbance

In Southland and Nelson there is a belt of rocks that
affects compasses measuring earth’s magnetic field. The
magnetic disturbance, known as the Stoke’s Magnetic Anomaly,
is caused by a band of rocks rich in iron and magnesium,
called an ophiolite belt. They are sections of old sea floor
that have become sandwiched in with the volcanic and
sedimentary Western Arc and Murihiku rocks. Even where these
rocks are not visible at the surface, magnetic disturbances
indicate where they are present deeper down.

The Haast schists

From about 200 million to 160 million years ago, as the
Western Arc, Murihiku and Torlesse sediments moved into the
coastal collision zone, the rocks at the base of the sediment
piles were undergoing a transformation. Pressure and heat
were breaking down the sediment, and it was recrystallising
to form glittering new rocks containing minerals such as mica
and garnet. These metamorphosed rocks are known as the Haast
schists. The mineral crystals within the different types of
schist are clues to the temperatures and pressures where they
were formed. To change Torlesse greywacke into the most
highly metamorphosed type of Haast schist, temperatures had
to reach over 300°C and the rocks had to be buried to a depth
of more than 10 kilometres.

Today, Haast schists form the bedrock in broad areas of
Otago and Southland, and are exposed in a narrow band along
the western edge of the Southern Alps. Some sections of ocean
floor caught in the collision were also metamorphosed,
forming rocks that include pounamu (New Zealand jade or
greenstone).

Cretaceous granites

Late in the cycle of mountain building, about 105 million
years ago (in the Cretaceous period), some rock within the
crust became hot enough to melt completely. The molten
material moved upwards, solidifying to form masses of granite
now found in areas such as Abel Tasman National Park.

Cretaceous granites often cannot be distinguished from
older granitic rocks by their appearance in outcrop.
Laboratory radiometric dating of minerals is needed to
determine the age of an igneous rock.

New Zealand breaks away from Gondwana

From highlands to lowlands

By 100 million years ago, in the middle Cretaceous period,
a region of new mountains stretched for several thousand
kilometres along the Australian and Antarctic margin of
Gondwana. These mountains were made of uplifted Western Arc
and Murihiku rocks, and the same grey Torlesse rocks that
make up today’s Southern Alps.

Erosion, however, immediately began to take its toll. By
the middle Cretaceous period, the mountains had largely worn
down to extensive lowlands. Lush vegetation mantled the river
flood plains and swamps of the low-lying region – this
greenery was eventually converted into the coal beds now
found in Otago, Southland, Westland and Nelson. Along the
coast and off the eastern shores of the new landmass, layers
of sediment eroded from the mountains built up. Preserved
within their layers were the remains of many creatures of
this period, including dinosaurs and marine reptiles such as
mososaurs and plesiosaurs.

Parting company

At the same time that erosion was wearing down the land,
patterns of circulation below the earth’s crust were
shifting. Hot rock began to well up beneath Gondwana and move
outward, pulling the land apart. A rift developed in
Gondwana’s crust, well inland of the coastal mountains. Along
this rift, molten rock rose to the surface, producing the
volcanic rocks now found in the Awatere and Clarence valleys,
and the Mt Peel, Malvern Hills and Mt Somers areas of
Canterbury.

By 85 million years ago, the sea had flooded into the
rift. A large section of Gondwana – including inland areas of
older rocks and the newer coastal region made of Western Arc,
Murihiku and Torlesse rocks – moved off into the Pacific
Ocean. New Zealand was now on its own – a drifting continent
about half the size of Australia. The new region of ocean
separating New Zealand from Gondwana became the Tasman Sea.
From dating the rocks that make up the floor of the Tasman
Sea, it is known that it took about 30 million years for it
to reach its present width.

Age of the dinosaurs ends

At the end of Cretaceous period, the impact of huge
meteors and large-scale volcanic activity resulted in the
extinction of about half the plant and animal species,
including the dinosaurs.

New Zealand rocks deposited at this time contain clues to
these global catastrophes. A thin layer of clay at locations
such as Woodside Creek in Marlborough contains high levels of
iridium, an element that is abundant in meteorites but rare
in normal rocks. The widespread destruction of forests is
indicated by abundant soot in the clay layer, and changes in
fossil pollen.

Going under

As the New Zealand continent moved away from the spreading
centre, its crust began to cool and become denser. The
low-lying land began to gradually sink into the ocean during
the early Tertiary period. By the Oligocene period, about 35
million years ago, less than one-third of the area of modern
New Zealand remained above sea level, as numerous
islands.

During this time, on the land still above the sea the bulk
of New Zealand’s coal deposits accumulated, including coal
now mined at Greymouth, Buller and Waikato. Offshore from the
islands a blanket of new sediments was laid down on top of
the older rocks of ancestral Gondwana. Sandstones and
mudstones were deposited close to shore. On shallow sea
floors far enough from land to be clear of sediment, the
calcareous remains of marine organisms built up, forming
large areas of limestones. These limestones are now used for
lime for agriculture, cement and Ōamaru’s famous building
stone. They have formed well-known scenic features such as
the Pancake Rocks at Punakaiki on the West Coast.

Today, parts of the North Island are still covered with
the layers of sedimentary rock that formed during this time.
In the South Island, however, most of this cover has been
eroded away, and in a few areas only patches have
survived.

New Zealand reborn

Sunken Zealandia splits up

About 25 million years ago, a shift in plate movements began to wrench apart the largely submerged New Zealand continent, Zealandia. In the north, sections of ocean floor of the Pacific Plate began to sink beneath continental rocks of the Australian Plate. Within the continent, pressure caused major cracks to develop. These cracks would eventually join to become New Zealand’s great Alpine Fault, splitting the continental mass in two. New Zealand now lay across two separate plates. These plates began to rotate. A sideswiping collision began, with the plates sliding past and running into each other. New land began to rise above the sea along the plate margins as colliding sections began to crumple. Volcanic activity and uplift increased, and substantial mountain building began about 5 million years ago.

Underwater basins

While many areas were being uplifted, parts of the New Zealand landmass were warped downward, creating large basin areas. As more land was pushed above the sea it began to erode and shed more sediment into the surrounding ocean. Layers of soft, grey mudstones and fine sandstones were deposited, with particularly thick accumulations along the east coast of both islands and in large subsiding areas such as the Taranaki and Whanganui basins along the North Island’s west coast.

Rocks of the Taranaki basin contain oil and natural gas derived from the organic material in the region’s older coal beds. The lighter gas and oil seeped upward, becoming trapped in the overlying layers of sediments that accumulated later in the Taranaki basin.

Uplift

As the land rose, the surface layers of younger rocks such as limestones, sandstones and coal were fractured and folded. In the rising ranges of the Southern Alps, however, most of the younger rocks were eroded away, exposing the underlying Torlesse rocks.

Papa rock

The soft sediments deposited during this period are locally known as papa or papa rock. Papa rocks have been uplifted and now make up many of the hill areas of the North Island. These soft rocks are prone to landsliding and are easily eroded during downpours, especially where the native forest has been cleared from steep hillsides to create pasture.

Building big volcanoes

During this period, volcanoes erupted in areas now far removed from current volcanic activity. Huge basaltic volcanoes formed the Banks and Otago peninsulas. Dunedin sits on the eroded remains of a volcano that first exploded to life about 13 million years ago. Volcanic activity continued there intermittently until 10 million years ago. Banks Peninsula is the eroded remnants of two large volcanoes. Lyttelton volcano began to erupt around 12 million years ago. It was later eroded, then partially buried by lava flows from the larger Akaroa volcano, which started building around 9 million years ago. Volcanic activity at Banks Peninsula finally died out around 6 million years ago.

The Coromandel Peninsula has seen numerous periods of volcanic activity, beginning around 18 million years ago and continuing to about 2.5 million years ago.

Quaternary mountains and glaciers

Forming the landscape

Immense changes which have occurred in the last 1.8 million years – the Quaternary period – have created the New Zealand landscape of today. The Southern Alps have risen thousands of metres, eruptions have created lofty volcanoes and buried large areas of the central North Island under rock, and huge glaciers have spread out from the mountains. During the Quaternary period, marine sediments continued to accumulate in coastal basins. Terrestrial rocks and sediments from this period cover the surface of much of New Zealand, and include coastal sand dunes, the sediment in river beds, and the scree on mountain slopes.

The Southern Alps

The uplifting of the Southern Alps has gradually accelerated, and today they are among the fastest-rising mountains in the world. Many of New Zealand’s mountain ranges have long straight fronts because blocks of bedrock are being pushed up along major faults. The highest rate of uplift is at the plate boundary, along the Alpine Fault. The land east of the fault is rising at average rates of 1–2 metres per century. The rock forming the summit of Aoraki/Mt Cook was below sea level less than a million years ago. In other areas the rock is being bent, crumpled and squeezed up. Erosion has kept pace with uplift, however, so the mountains have rarely been much higher than they are now. Rivers, glaciers and gravity have, during the Quaternary period, carved out the entire landscape we see in the Southern Alps.

Glaciers

About 2.6 million years ago, a little before the start of the Quaternary period, earth plunged into cycles of repeated climate cooling known as ice ages. During glacial periods, average temperatures dropped by as much as 4.5°C, and lots of heavy snow fell on New Zealand’s high mountains. The steadily accumulating snow hardened into ice, forming huge glaciers that moved downhill into lower regions. At the height of glacial periods, glaciers blanketed the mountains from Fiordland to west Nelson, with smaller glaciers in the North Island’s Tararua and Ruahine ranges and on the central volcanoes.

Glaciers act as giant conveyor belts, moving rock debris from the mountains to lowland areas, and dumping it in great ridges, called moraines, along the flanks and front ends of the glaciers. When the glaciers later melted, these ridges were left, outlining the former extent of the ice. On the South Island’s West Coast, moraine ridges hundreds of metres high extend down to the coast and out under the sea. In the eastern South Island, remnants of moraines indicate that ice once reached the top of the Canterbury Plains. Rock debris carried by the glaciers was also flushed down rivers, filling river valleys with thick gravelly deposits.

During the ice ages, massive glaciers and ice caps formed and retreated many times worldwide. Few deposits of early glaciations survive in New Zealand – they were usually overrun and destroyed by glaciers during later advances. In addition, in the rapidly rising mountains, glacier debris tended to be quickly eroded away by rivers.

Ice-carved land

All of the ice-sculpted landforms of the Southern Alps are the product of advances and retreats of the ice in the South Island during the last 250,000 years. The most extensive moraines are from the most recent glaciation, the Ōtira Glaciation, which reached its maximum around 18,000 years ago. As ice has retreated, the depressions behind some of these moraines have filled with water, creating some of New Zealand’s most scenic lakes, such as Te Anau, Wakatipu, Tekapo and Pūkaki.

Quaternary coasts and rivers

Rivers – moving mountain debris

During glacial periods over the last two million years,
the rivers of the South Island have carried very large loads
of debris dumped into them by glaciers. The river valleys
have filled with thick layers of gravels. During periods when
the rivers carry less sediment, or where the land has been
raised by tectonic activity, the rivers have cut down and
removed much of the gravels. The remains of the gravels form
flat-topped terraces flanking the river valleys well above
the level of the present riverbeds.

Rivers – building plains

New Zealand has a remarkably rugged landscape – the rapid
uplift of the land to form mountains has left it with few
flat areas. The most extensive lowlands, such as Hawke’s Bay
and the Canterbury Plains, have been created by rivers
depositing vast quantities of sediments eroded from upland
areas. As rivers have emerged from the confines of valleys,
they have dumped their sediment load, forming great spreading
fans of sand and gravel. For example, through many glacial
and interglacial periods, the Canterbury Plains have built
outward from the front of the Southern Alps, eventually
reaching Banks Peninsula, which was once a volcanic offshore
island.

Wind-blown dust is a common sight along New Zealand’s huge
gravel-bed rivers, and during glacial advances the rivers
were carrying even more fine material ground up by the
glaciers. Thick layers of this dust, known as loess, have
accumulated in many areas of the New Zealand landscape.

Sea-level ups and downs

During the Quaternary period, as huge ice caps built up in
the northern hemisphere, water became locked up in the ice
and the sea level dropped by more than 100 metres. When the
ice caps later melted, water was returned to the oceans and
the sea level rose.

A single island

At the height of the last glaciation, about 20,000 years
ago, areas of sea floor at the shallow northern end of Cook
Strait were above sea level. It would have been possible to
walk between the North and South islands.

When the sea was lower during glacial periods, more of the
continental shelf was exposed and the coastline was seaward
of its present position. When the sea rose, it flooded back
in over the coastal land. Different types of sediment have
been deposited. The Canterbury Plains have alternating layers
of porous gravels deposited by rivers when the sea was low,
and finer impermeable sediment laid down when the sea level
was high. These layers trap water and are responsible for the
region’s excellent artesian water supply.

Where hilly terrain meets the sea, waves cut into the
hillsides, creating cliffs with flat beach platforms at their
base. The beach platforms formed during periods when the sea
level was higher were left as terraces far above the ocean
when the sea level dropped. Other beach platforms were raised
by movement of the earth’s crust. These marine terraces are
common around rugged sections of the coast.

Quaternary volcanism

The last two million years (Quarternary Period) have been
marked by violent volcanic activity in the North Island,
where pockets of molten rock have welled up to the
surface.

Super-volcanoes

The largest and most violent volcanoes in New Zealand are
not cone-shaped mountains; they are huge basin-shaped
volcanic depressions known as calderas. New Zealand has a
number of these super-volcanoes, including the Taupō,
Rotorua, and Okataina calderas. Created by repeated
catastrophic eruptions during the last 1.6 million years,
some calderas are now occupied by lakes, such as at Taupō and
Rotorua.

During caldera eruptions, magma is blasted out largely in
rapid flows of incandescent ash, pumice and gases. When the
material comes to rest, it forms a rock known as ignimbrite.
Plateaus of ignimbrite hundreds of metres thick surround the
calderas. Ash from their eruptions has spread for thousands
of kilometres – for example, ash from the Taupō caldera is
found in the Chatham Islands.

Most of New Zealand’s geothermal areas, such as Rotorua
and Waimangu, lie within the calderas. Deep molten magma
provides the heat that keeps the geysers, hot springs and mud
pools bubbling.

Andesitic volcanic cones

The major active volcanoes of the North Island include Mt
Taranaki (Mt Egmont), the peaks of Tongariro National Park
(Tongariro, Ngāuruhoe, Ruapehu) and White Island. Eruptions
began in the Taranaki area around 1.7 million years ago. The
volcanoes of Tongariro National Park have been built largely
during the last 260,000 years.

Auckland – a city on volcanoes

Auckland is built on a volcanic field that has been active
as recently as 600 years ago. Scattered through the city are
dozens of volcanic cones. The oldest, Maungataketake, is
about 50,000 years old, and the most recent volcano is
Rangitoto Island. The eruptions that built each cone have
been short-lived, spanning perhaps as little as 10 years.

Volcanic timelines

The numerous eruptions that have spread volcanic ash far
and wide over the New Zealand landscape during the Quaternary
period have created unique time markers. Volcanic ash
(tephra) from individual eruptions can be identified by their
distinctive compositions and dated by radiometric methods.
The ash layers can often be found within beds of other
sediment – any sediment on top of the ash must have been
deposited after the ash was erupted. Ash layers have been
used to date landscape features such as deposits left by ice
age glaciers.

Holocene – the last 10,000 years

The Holocene period, covering the last 10,000 years, is
characterised by a warm and relatively stable climate.
Nevertheless, the pace of uplift and land deformation,
dictated by the movement of the Pacific and Australian
plates, has ensured that geological change within New Zealand
has remained extremely rapid.

Warming climate, rising seas

Beginning about 18,000 years ago, earth started to emerge
from the latest glacial period. As the climate warmed, water
from the planet’s ice caps poured back into the oceans and
their level rose. Sea level reached a maximum at about 7,000
years ago (about 5000 BC), and has remained stable since
then.

The rising sea flooded low-lying areas, and in many places
in New Zealand, extended well inland of the present
coastline. Since then the coast has built outwards in many
places, as sediment eroded from the land has been carried by
rivers to the ocean. Waves and currents have distributed this
sediment along the coast. Long spits have formed, connecting
areas of land and building across the heads of bays, and
forming features such as Farewell Spit and the Kaitorete
barrier beach damming Lake Ellesmere.

Volcanoes

Activity in the central North Island’s volcanic zone has
continued unabated into the Holocene period. About 1,800
years ago the Taupō caldera unleashed the most powerful
volcanic eruption on earth in the past 5,000 years,
incinerating over a sixth of the North Island. Tongariro,
Ruapehu and Taranaki have erupted intermittently, spreading
fine ash over the surrounding countryside, and the striking
volcanic cone of Ngāuruhoe has been built entirely within the
last 2,500 years.

During the Holocene period several volcanic cones have
been added to Auckland’s skyline, including Mt Wellington.
The formation of Rangitoto was witnessed by Māori about 600
years ago.

Deformation of the land

Accompanied by frequent earthquakes – mostly small but
sometimes destructive – the continent of Zealandia is being
compressed along the boundary between the Australian and
Pacific plates. Squeezed between the converging plates, the
crest of the Southern Alps is currently rising at an
estimated rate of 10 millimetres per year. This rise is not
smooth – in large earthquakes, blocks of land may rapidly
lurch several metres upward. Sections of countryside may also
move many metres horizontally – in the magnitude 8.2
earthquake of 1855, land shifted up to 18 metres along the
Wairarapa Fault.

Not all of New Zealand is rising. The north-east corner of
the South Island, on the edge of the Taranaki basin, is
subsiding. Here the sea has flooded into former river
valleys, creating the maze of waterways of the Marlborough
Sounds.

Humans

The Holocene period is sometimes known as the Anthropogene
or the Age of Man. Although our own species, Homo
sapiens, appeared well before the start of the Holocene,
this period encompasses all of humanity’s recorded history.
New Zealand was one of the last land areas in the world to be
settled by humans, with the arrival of Māori about 1250–1300
AD and Europeans from about 1790.

Humans have had an immense impact on the natural
environment of New Zealand in only 700 years. Forest-burning
and conversion of land to agriculture has removed over
two-thirds of the native forest, causing major ecological
changes and accelerating erosion. Hunting and the
introduction of alien plants and animals has led to the
extinction of native species. Future geologists will
recognise the impact of humans in New Zealand as a sudden
change in the record of rocks and fossils.

External links and sources

More suggestions and sources

Coates, Glen. The rise and fall of the Southern Alps. Christchurch: Canterbury University Press, 2002.

This classic paper by D. S. Coombs, published in the Transactions of the Royal Society of New Zealand in 1954, documents the mineralogical changes with burial of a thick sequence of sediments in the Taringatura Hills.

How to cite this page: Eileen McSaveney and Simon Nathan, 'Geology – overview', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/en/geology-overview/print (accessed 19 December 2018)